1. Field of the Invention
The present invention is related to passive electrical elements and, more particularly, to passive electrical currents used in an IF filter for a wireless communications system.
2. Background Description
Wireless communications is evolving to allow individuals access to information as well as contacting other people, anywhere, anyplace, and at anytime. Generally, examples of state of the art wireless communications systems include FM radio, television, personal communications systems, wireless PBXs, wireless local area networks (LANs) and mobile or cellular telephone communication networks. These new communications systems and devices are dramatically changing society and allowing workers to become xe2x80x9cuntetheredxe2x80x9d from their information sources.
The most common wireless communications system receiver is called a super-heterodyne receiver. Super-heterodyne receivers, which are well known in the art, receive a communications information signal on a radio frequency (RF) carrier that is combined with a slightly lower frequency to generate a sum and a difference signal. The communications information signal is included on each of the sum and difference signals. The higher frequency sum signal (which may be at a frequency nearly twice that of the RF input) is filtered out leaving only the difference or intermediate frequency (IF) signal. The intermediate frequency is also called the beat or heterodyne frequency. The IF signal is between RF and the output signal band, which is typically in the audible frequency range. The IF signal is further filtered, amplified and then, passed to a detector which extracts the input signal to produce the desired audio or other output signal.
FIG. 1 is an example of a typical state of the art super-heterodyne wireless receiver 50. Input RF at an RF antenna 52 passes to a ceramic filter 54. The ceramic filter 54 filters the received RF from RF antenna 52 and passes the filtered RF to a RF low noise amplifier (LNA) 56. The output of the RF low noise amplifier 56 is passed to a mixer 58. Mixer 58 combines an oscillator frequency from a voltage control oscillator (VCO) 60 with the amplified RF output of the low noise amplifier 56 to provide sum and difference signals, the difference signal being the IF signal. The sum and difference signals are passed to a surface acoustic wave (SAW) filter 62. SAW 62 is an intermediate filter that filters out the sum signal, passing only the IF signal. The IF signal is passed to a second low noise amplifier 64. The amplified IF output of the second low noise amplifier 64 is passed to a second mixer 66. Second mixer 66 mixes the output of the second low noise amplifier 64 with a pure IF carrier from a second VCO 68. A crystal oscillator 70 provides a base frequency to the first VCO 60 and the second VCO 68. The output of the second mixer 66 is passed to an IF crystal filter 72. The filtered output of the IF crystal filter 72 is passed to broadband electronics.
Thus, it can be seen that all active devices (i.e., transistors) used to form the low noise amplifiers 56, 64, mixers 58, 66 and VCOs 60, 68 may be on a typical integrated circuit. The typical low power output from a state of the art passive IC device is susceptible to signal loss both from stray capacitance and inductance, as well as, ambient noise. Consequently, most of an output signal from a passive IC device (e.g., an inductor or a capacitor) may be lost on a long interconnecting wire, for example, when a capacitive pick-up is used in a microsensor. Unfortunately, the ceramic RF filter 54, IF SAW filter 62, crystal oscillator 70 and IF crystal filter 72 are not easily integratable with the other circuits using state-of-the-art integration techniques. Instead, discrete passive elements (capacitors, inductors and resistors) forming components 54, 62, 70, 72 require much more space and power than is currently available from typical state of the art solid state circuit technologies. Discrete component elements are subject to ambient conditions and variations in ambient operating conditions and so, tend to degrade system level performance. Also, because of the high frequency (RF) use of these passive components, they must be shielded. The shielding both prevents RF noise from being coupled inadvertently to the circuit, especially in inductors, as well as blocks unwanted RF radiation from the components. However, the RF shielding further increases the size of these RF passive components.
An alternative to a superheterodyne receiver that does not include an IF stage is a direct conversion receiver (DCR). Because a DCR does not have an IF stage, it does not have many of the passive components of the superheterodyne receiver. However, foregoing passive components in state of the art DCRs comes at a cost of degraded performance because state of the art DCRs require a local RF oscillator at the same frequency as the RF input signal. The DCR directly mixes an input signal from its antenna with an RF at carrier frequency from the local RF oscillator. Noise from the local RF oscillator can feed back through the receiver and antenna to the transmitter, reducing receiver sensitivity and interfering with other wireless communications devices, especially those in close proximity to the particular DCR.
Thus, there is a need for a low cost fully integrated super-heterodyne receiver. Further, there is a need for a way to include passive elements and especially resonant passive elements on integrated circuits.
It is a purpose of the invention to include RF filters, resonators and other high frequency passive devices onto the same integrated circuit chips with circuits driving/being driven by those high frequency devices.
The present invention is an electromechanical structure such as a MEMS resonator formed in the surface of a semiconductor body. A flexible beam containing a conductive plate is integrally formed in a cavity in the surface of a semiconductor body. A second conductive plate is parallel to the first along a sidewall of the cavity. A voltage applied across the first and second conductive plates forces the flexible beam to vibrate horizontally. A cap layer seals the cavity and maintains a vacuum in the cavity. The structure is smaller than the wavelength of the RF signal generated therefrom and, therefore, virtually shielded.